GPS attitude determination systems use GPS phase measurements to determine the orientation of a baseline defined by two antennas. Because antenna separation is generally greater than one wavelength, there is an integer ambiguity for each satellite that must be determined before the orientation of the baseline can be calculated. Since for attitude determination the baseline length is precisely known, the initial integer ambiguity search seeks to identify all solutions that result in a baseline length close to the true baseline length.
Integer cycle ambiguity resolution is a problem commonly encountered in GPS applications such as attitude determination and kinematic positioning. There are many recent papers on the design and performance of ambiguity resolution algorithms for kinematic positioning. Ambiguity resolution for attitude determination is different from ambiguity resolution for kinematic positioning in that for attitude determination the baseline is generally relatively short and the baseline length is very precisely known. Therefore, for attitude determination systems, the initial integer cycle ambiguity search is generally performed in solution space instead of measurement space. In other words, the initial search is based on baseline length instead of on the phase residual.
It has been shown in several recent papers that instantaneous ambiguity resolution is often possible for attitude determination systems. There are two principal design issues for ambiguity resolution: (1) Determination of the smallest possible search space from all available information and design requirements; and (2) Efficient identification of all solutions that lie in the search space. Due to the fundamental structure of the problem, an instantaneous ambiguity resolution algorithm cannot guarantee that it will produce exactly one solution, and even when exactly one solution is produced, it cannot guarantee that the solution is correct.